your not alone as many have been caught out by this dvla failure fine over the years, in most cases the system will work but for whatever reason if the V5 fails to reach their office, theres not a lot you can do but appeal but if they don't accept your reasoning and reject it chances are you have to suck it up, for the sake of a extra 75p to record something its a safe practice to always send any official documents via that method as a minimum